Batteries and fuel cells are electrochemical cells used to generate an external electrical current. They consist of an anode, where oxidation occurs, a cathode, where reduction occurs, and an electrolyte through which ions can travel between electrodes (see Figure 1 for a schematic of a common battery cell). In fuel cells (discussed below), one or both of the reactants are supplied from an external source to the cell. Though technically fuel cells, if the only reactant supplied to the cell is atmospheric oxygen, the cells are then considered batteries (zinc/air or aluminum/air cells for example).

Figure 1. Schematic for an electrochemical cell.

Primary and Secondary Batteries and Their Differences

Batteries can be divided into two types: primary or disposable batteries and secondary or rechargeable batteries. The main advantages of batteries over fuel cells are their availability, portability, low cost, and wide range of operating conditions. Batteries, however, have much shorter life spans and lack the power output of fuel cells. Power outputs of batteries are typically on the order of 100's of watts, whereas fuel cells can provide kilowatt to megawatt outputs, power enough to light a building or fuel a vehicle for hours. Under heavy energy demands, batteries can build up dangerous levels of heat and pressure, degrading the battery and possibly causing leaks of toxic compounds or even explosions. In addition, the limited life of primary batteries and the limited cycle life (number of times it can be recharged) of most secondary batteries necessitate the need for disposal of often dangerous and toxic battery materials. Table 1 summarizes some of the common types of primary and secondary batteries.

Table 1. Common battery types.

	Battery Type	Anode	Cathode	Electrolyte
Primary Batteries	Alkaline Cell	Zn	MnO2	KOH
	Aluminum/Air Cell	Al	O2	KOH or neutral salt solution
	Leclanché Cell (Zinc Carbon or Dry Cell)	Zn	MnO2	NH4Cl or ZnCl2
	Lithium Cell	Li	Various liquid or solid materials	SOCl2, SO2Cl2, or organic solutions
	Mercury Oxide Cell	Zn or Cd	HgO	KOH
	Zinc/Air Cell	Zn	O2	KOH
Secondary (rechargeable) Batteries	Iron Nickel Cell	Fe	Ni(OH)2	KOH
	Lead/Acid Cell	Pb	PbO2	dilute H2SO4(aq)
	Lithium Ion Cell	C, carbon compounds	Li2O, intercalated into graphite	LiPF6, LiBF4, related compounds
	Nickel/Cadmium Cell	Cd	Ni(OH)2	KOH
	Nickel/Metal Hydride (NiMH) Cell	Lanthanide or Ni alloys	Ni(OH)2	KOH
	Nickel/Zinc Cell	Zn	NiO	KOH
	Sodium/Sulfur Cell	Molten Na	Molten S	Al2O3

Table 1 (cont). Common battery types.

	Battery Type	Advantages	Disadvantages
Primary Batteries	Alkaline Cell	High energy density, long shelf life, good leak resistance, performs well under heavy or light use.	Costlier than zinc-carbon cell but more efficient
	Aluminum/Air Cell	Can operate exposed to sea water (neutral salt solution), easily replaceable electrolytes/electrodes	Anode quickly degrades, short shelf life, short operational life
	Leclanché Cell (Zinc Carbon or Dry Cell)	Cheap and common (oldest available battery type)	Poor performance under heavy or continuous use.
	Lithium Cell	Very high energy density, long shelf life, long operational life	Poor performance under heavy use, vulnerable to leaks or explosions
	Mercury Oxide Cell	Higher energy density than (Zn/MnO2) alkaline cell	High cost and being phased out due to toxicity concerns
	Zinc/Air Cell	Environmentally benign, cheap, very high energy density, and virtually unlimited shelf life	Short operational life, low power density
Secondary (rechargeable) Batteries	Iron Nickel Cell	Long life under a variety of conditions, excellent back-up battery	Low rate-performance, slow recharge rate
	Lead/Acid Cell	Low cost, long life cycle, operates well under a variety of conditions. Common car batteries	Minor risk of leakage
	Lithium Ion Cell	Relatively cheap, high energy density, long shelf life, long operational life, long cycle life	Minor risk of leakage
	Nickel/Cadmium Cell	Good performance under heavy discharge and/or low temperature	High cost, can temporary loose cell capacity if not fully discharged before recharging (memory effect)
	Nickel/Metal Hydride (NiMH) Cell	High capacity and power density	High cost, some memory effect
	Nickel/Zinc Cell	Low cost, low toxicity, good for high discharge rates	Zinc on the electrolyte tends to redeposit unevenly on anode, severely reducing efficiency
	Sodium/Sulfur Cell	Inexpensive materials, long cycle life, high energy and power	High operational temperature lower efficiency, some danger of explosion upon degradation

Component Materials in a Battery

The primary component materials of a battery are the anode, cathode, electrolyte, and semi-permeable materials. In addition various catalysts have been used to enhance the performance of electrodes. For example, ruthenium(IV) oxide (238058) is used as a catalyst in a vanadium redox battery system. Table 1 summarizes some of the types of electrodes and electrolytes used in common batteries. Many advanced battery designs focus upon new materials for these key components.
Current Research Areas in Battery Development

Much of the recent battery work has focused on lithium-ion batteries, since they are the primary power source for the ever-growing field of small, rechargeable electronic devices. Nickel sulfide (343226), for example, was recently explored as a cathode material for rechargeable lithium batteries. Current research is also concerned with some very mundane materials in electrodes. New morphologies of graphite flakes, as a case in point, have been studied as anode material in lithium-ion batteries. Electrolytes are also very important in battery performance. A lithium tetrafluoroborate (LiBF4 255815) solution, for example in a butyrolacetone/ethylene carbonate solution has proven to be a highly conductive and highly thermally stable electrolyte for lithium-ion batteries.
High Purity Inorganics

Sigma-Aldrich maintains the highest standards for quality control and quality assurance. High-purity materials are rigorously analyzed by a variety of techniques including trace metals analysis by ICP, which can detect impurities an order of magnitude below ppm levels. Fuels cells and batteries often require high purity components. For example, the electrolytes in low-temperature rechargeable batteries can be from alkyl carbonates and high purity lithium salts of the form LiEF6 (E = P, As).

High purity inorganics also find significant industrial usage. More than 60% of the industrially used cadmium is in Ni-Cd batteries, of which 75% is found in cellular phones. Much of the remainder of this portion is also used in the telecommunications industry as materials in emergency power supplies for electronic telephone exchanges.
Liquid Electrolytes

The type of electrolyte used for a fuel cell depends upon the choice of fuel cell (see Table 1). The key role of the electrolyte is to create a medium through which ions can move between the anode and the cathode. Electrolytes can also act as a kind of filter, preventing undesirable ions or electrons from disrupting the desired chemical reactions.
Plasticizers and Binders

The use of plasticizers in commercial polymer formulations to decrease Tg and the internal viscosity, and to increase bulk flexibility is a well-established practice in a multitude of industrial applications. In fact, the “new car smell” enjoyed by many car owners results mainly from the phthalate plasticizer vaporized in the closed car interior, and actually advertises the deterioration of the vinyl upholstery. To improve the permanence of the plasticizer higher-molecular-weight phthalates are commonly used for modern car interiors. A number of criteria are considered in choosing a plasticizer, including cost, compatibility, stability, ease of processing, and permanence. In addition to the aforementioned uses, a growing body of research has emerged over the past two decades on the application of plasticized polymers in areas that involve properties not usually associated with polymers. For example, the introduction of oligomeric poly(ethylene glycols) (PEG) and derivatives as plasticizers, to effect a significant increase in ionic conductivity as solid polymer electrolytes (SPEs), for use in high energy density batteries and other solid-state electrochemical devices.

Cellulose triacetate membranes, plasticized with 2-nitrophenyl octyl ether, are used as materials for separations. They are impermeable to metal cations, but allow anion

Exchange20 and are also remarkably permeable to neutral, mono- and disaccharides. Highly efficient photorefractive polymer composites can be formed using 9-ethylcarbazole (ECZ) as a plasticizer in guest-host polymers.